In traditional manufacturing of fibre boards, so-called soft- and hardboards, fibre mats to be transformed into a finished board are formed in a wet process utilizing natural binding mechanisms of wood cells to establish a binding of the fibres. The finished boards are produced in a hot pressing process from these fibre mats, fibre boards are often also referred to as fibre panels or fibre plates or simply panels or plates.
For especially environmental reasons, this process has been replaced by a dry process over the last 2-3 decades. In this process, a new product called Medium Density Fibreboard—MDF—is made by pressing a mat of dry fibres with a moisture content about approximately 10%, i.e. usually 10%±3.
Unlike the wet process, the dry process does not allow for utilizing the natural binding mechanisms of the wood cells. Instead a thermosetting synthetic binder, usually a urea-formaldehyde or a melamine-formaldehyde condensate or a mixture of both or, for special products, polyurethane or isocyanate, is added to replace the natural binding mechanisms, usually in a fluent, water-diluted form. The application of the synthetic binder is typically done according to 2 basic principles,
1) Mechanical blending employing a cylinder housing and a rotating blending device. Fibres and binder are fed into one end of the cylinder and the blending device mixes the components and moves the mixture through the cylinder to allow a continuous process. This method, which was adopted from particleboard manufacturing, has one disadvantage: The mixing is not sufficiently homogeneous, whereby fibre lumps with a high percentage of binder produced finished panels having hard and dark “glue spots”.2) An airborne method called the blow-line method (which replaced mechanical blending), containing the following process steps:Wood chips are milled into fibres in a so-called disc refiner and exit the refiner periphery through a tube called the blow-line at a velocity in the range of 100-300 m/sec. Within the blow-line an aqueous solution of the binder is added at high pressure. Combined with the high speed flow of fibres and steam, the binder infeed functions as a two-phase nozzle.
The mixing of the rather large wet fibre lumps (˜100% moisture content) and the binder is not very intense in this stage of the process but as the fibre and resin mixture is led into a flash dryer tube (cross section typically 200 times larger than the blow-line), the fibre lumps are eddied apart by turbulence. During the transport through the flash dryer at low speed (10-30 m/sec.) an intense mixing of fibres and binder takes place. In addition to the mixing, drying the fibre-binder mixture to a moisture content about approximately 10%, i.e. usually 10%±3, of dry matter is obtained.
The blow-line method has the advantage over the traditional blender mixing that it produces less glue spots in the final product. However, it has some serious drawbacks:                When an aqueous solution of binder is applied to the wet fibre, a large proportion of the resin is absorbed by the fibre during the subsequent drying process. Consequently, this part of the resin is not useful in establishing a proper bonding between the fibres during the later hot pressing process, i.e. more binder is needed.        Travelling through the dryer tube with an initial temperature in the range of 180-200° C. and a final temperature in the range of 60-80° C., the binder has partly been cured and lost at least some of its binding effect, i.e. more binder is needed.        To counteract this effect, slow-curing binders are used. However, as a consequence, longer press times in the hot press are needed in order to activate the binder.        
Blow-line application of the binder is a costly compromise, dictated mainly by requirements to the surface quality of the finished product. Consequently, less disadvantageous methods of binder application have been sought after.
One approach is a reconsideration of the traditional blender method from the 1970s.
More advantageous approaches are based on the idea of applying the binder in an airborne process after the dryer, since:                Applying the binder to the dry fibres prevents pre-curing of the binder during the process, i.e. less binder is needed.        Applying the binder to the dry fibres provides less absorption of binder into the fibre surface, i.e. a better bonding efficiency of the binder droplets and less binder needed to achieve a specific bonding quality.        Further, this effect can be enhanced by regulating the dry content of the binder solution, which has no effect in the blow-line process.        
As pre-curing of the binder does not limit the temperature in the flash dryer tube, the fibre drying can be made at much higher temperatures, e.g. an inlet temperature of up to 400° C. or higher as used in the particle board industry. As a result, an increased capacity and a more efficiently controlled drying process can be obtained.
Drying the fibre-binder mixture in the blow-line process causes substantial emission of formaldehyde from the synthetic binder, usually a urea-formaldehyde condensate. Costly measures to solve this problem are not needed if the binder is applied to the dry fibres.
The problems to be overcome when applying the binder at this stage of the process, however, are very substantial.
Due the chemical composition of lignocellulosis biomass fibres and the dipole moments in relation hereto, the fibres tend to agglomerate to lumps, especially when dry.
To achieve a homogeneous distribution of the binder droplets in a device used in the process after the dryer, these fibre lumps are to be separated into single fibres.
At the same time, the binder preferably has to be atomised into droplets of a proper size in relation to the size of the fibres and they have to be brought into contact with the fibres to ensure a homogeneous distribution on the fibre surfaces.
Besides, the binder droplets preferably have to have a specific viscosity to adhere sufficiently to the fibre surfaces without becoming fully absorbed, and they must be prevented from sticking to the walls of the device.
Unlike the blow-line application of binder, the dry application of binder after the flash dryer does not offer the opportunity of homogenizing the mixture during the long travel through the dryer.
Therefore all the above mentioned conditions are to be satisfied within little time and space.
Various attempts have been made to overcome the difficulties of meeting these requirements.
Patent specification DE 101 53 593.7 pays attention to the above mentioned problems of establishing a homogenous airborne flow of fibres in a so-called transportation tube at a high air velocity (>20 m/sec.). From this tube, the fibre flow is fed by a nozzle into the bottom section of a vertical tower of much larger diameter. The fibre lumps are separated by the turbulence in the area around the nozzle, and the slow, upward air flow ensures that agglomerated fibre lumps sink to the bottom of the tower.
Binder is sprayed upwards the fibre flow at various positions over the height of the tower, and the contact between fibres and binder droplets is facilitated by grounding the binder supply and by using special materials in the tubes to establish an electrostatic load on the fibres by friction.
An equipment according to this method has been established and is supposed to function satisfyingly. The problems in relation to fibres and binder sticking to the walls of the equipment are apparently not solved. However, patent specification EP 1 398 127 A1 describes a procedure for periodical cleaning of the walls of the tube.
Establishing a zone of turbulence to separate the fibre lumps into single fibres is the vital part of other patent applications, too.
Patent specification DE 199 30 800 describes a binder application device to be installed at the outlet of a flash dryer tube. The diameter of the cylindrical binder application device is much larger than the flash dryer tube, whereby turbulence at the inlet of the device is expected to separate the fibre lumps. This effect is supported by the compressed air used to spray the aqueous solution of binder at the inlet of the device.
Special attention is led to the problem about binder and fibres sticking to the walls of the device. This problem is dealt with by means of compressed air led through a large number of orifices in the walls of the device, creating a protective mantle of air turbulence along the walls of the device.
A similar solution of the problem of binder and fibres sticking to the walls of a tubular device when applying an aqueous binder solution to the dry fibres has been used in patent specification EP 102 21 03, employing a double-wall cylinder construction to guide an air stream through a multitude of drillings in the inner wall to create a protection mantle of air and thus to prevent fibres and binder to adhere to the wall. However, in terms of achieving a homogeneous mixture of single fibres and binder droplets no non-prior art information is disclosed.
Handling of fibre flow in order to create a flow of single fibres is also a central part of patent specification U.S. Pat. No. 5,827,566. Turbulence to separate the fibre lumps into single fibres is achieved by inserting a device containing a tube section with a reduced cross section (a Venturi nozzle) to accelerate the flow followed by a bulge with a large diameter (a diffuser), where by means of turbulence the fibre lumps are separated and an aqueous solution of binder is sprayed into the fibre flow.
The proposal of cooling the walls in the diffuser to prevent binder and fibres to stick to the wall is a traditional technique used in mechanical blenders in the particle board industry and thus prior art. This also applies to the proposal of heating the binder solution e.g. to a temperature of 60° C. to ensure low viscosity and good spraying properties with a low percentage of water.
While all patents and patent applications quoted above are based on an airborne transportation of fibres into the binder application device, patent specification DE 197 40 676 employs a cylindrical tower, into which the fibres are fed mechanically into an upper end of the tower and move downwards through the tower only by gravity at low speed, while a binder solution is sprayed onto the fibres. Remaining fibre agglomerates are preferably separated mechanically, using a disc refiner set to a distance between the discs to only influence the fibre lumps by turbulence.
In previous patents and patent applications, methods are disclosed to handle important questions in relation to applying the binder solution on to fibres after drying, i.e. how do we separate the fibre lumps into single fibres ?, how do we ensure that binder droplets of the optimal size are brought into close contact with the fibres ?, and how do we prevent the mixture to stick to the walls of the device ?
Equipment using turbulent air flow to rip the fibre lumps apart are predominant in known methods.
In the following, a novel method based on a different kinetic technique and an equipment to handle the fibres and binder droplets will be disclosed.